The present invention relates to an actuator of the type wherein an electric motor drives an activation element, such as a rotatable arm or a longitudinally movable rod, via a transmission, and wherein the transmission has at least two transmission stages, the first one being formed by a worm drive with a worm and a worm wheel.
Actuators are used inter alia for performing movements and positional adjustments in articles of furniture, including beds and loose mattress supports, sickbeds and hospital beds, nursing equipment, seats for vehicles, machinery and within the industry, etc. Examples of linear actuators may be found in the applicant""s EP 647 799 and EP 662 573, both Linak A/S, and DE 38 42 078 Niko Gesellschaft fxc3xcr Antriebstechnik GmbH as well as WO99/16333 Inventions and Developments Holland B.V. An example of a rotary actuator is found in German Utility Model DE 298 02 384 U1 Recticel Internationale Bettsysteme GmbH, which discloses a mattress support with incorporated rotary actuators for adjusting the backrest part and/or the leg-rest part.
The associated electrical equipment is usually a low volt equipment where the actuators are typically driven by a 24V DC motor. The motors available involve the problem that a strong gearing has to take place, as the motors have a relatively high number of revolutions, while the actuator has to move very slowly relatively to this. For example, the motor may have a number of revolutions of the order of 3000 r.p.m., while the actuator performs a full movement in a period of time of the order of 30 sec. or even as slow as 60 sec. Various solutions have been contemplated in order to achieve the high reduction, but these have either been too expensive, too bulky, not sufficiently mechanically stable or too noisy.
The object of the invention is to provide an actuator which allows one or more of these problems to be obviated.
This is achieved according to the invention with an actuator of the type stated in the opening paragraph, wherein the subsequent transmission stage is formed by a crown and bevel wheel, wherein the bevel wheel is driven by the worm wheel, and wherein the crown wheel has a power take off arranged in extension of, or substantially in extension of, the worm, or in parallel therewith. This provides an actuator having a relatively compact transmission with a high gearing, and with a worm wheel made of plastic, the actuator is moreover comparatively noiseless. Crown and bevel wheels may be manufactured with simple moulding tools, by plastics moulding with a two-jawed tool, so that basically the manufacturing costs may be kept low.
When the transmission comprises an additional worm wheel and bevel wheel arranged diametrically opposite the first worm and bevel wheel, a balanced distribution of the forces is achieved. The worm and the crown wheel are stabilized, and the force distribution may be used for transferring greater forces or reducing dimensions or alternative selections of materials.
In a preferred embodiment the worm wheel and the bevel wheel are formed as one piece, which ensures optimum coupling and also facilitates mounting. The worm (worm gear) and bevel wheels may be made as one plastic member in a two-jawed injection moulding tool.
In a further embodiment the motor housing is provided with a front cover with a mount for the free end of the worm, and the cover is additionally provided with shafts for worm wheel/bevel wheel as well as a shaft for the crown wheel. This has evident advantages in terms of mounting.
When the power take-off of the crown wheel is formed as an axis-parallel toothed wheel, it may readily be used as a sun wheel when built together with a planetary gear. Here, too, it is expedient that the crown wheel and the toothed wheel are made as one piece.
In an embodiment, the shaft of the crown wheel is provided with a bearing bushing, and also the free end of the worm is expediently mounted in a bearing bushing in the front member.
When constructing worm wheel, bevel wheel and crown wheel, the crown having a toothed wheel as power take-off, of plastic, the costs may be reduced while keeping the noise t a low level.
The invention will be explained more fully below with reference to the embodiments illustrated in the accompanying drawings.